1. Field of the Invention
The present invention relates to a display device for displaying images by input of video signals, and more particularly to a display device having light-emitting elements. In addition, the invention relates to an electronic appliance using the display device.
2. Description of the Related Art
Description is made below on a display device for displaying images by disposing a light-emitting element in each pixel and controlling the light emission thereof The display device has a display and a peripheral circuit for inputting signals thereto. FIG. 16 shows a configuration of a pixel portion of a display.
In a pixel portion 1603, source signal lines S1 to Sx, gate signal lines G1 to Gy, power source lines V1 to Vx, and pixels having a matrix arrangement of x (x is a natural number) columns and y (y is a natural number) rows are disposed. Each pixel 1700 includes a switching transistor 1701, a driving transistor 1702, a capacitor 1703 and a light-emitting element 1704.
FIG. 17 shows an enlarged view of one pixel in the pixel portion 1603 shown in FIG. 16.
The pixel includes one source signal line S among the source signal lines S1 to Sx, one gate line G among the gate signal lines G1 to Gy, one power source line V among the power source lines V1 to Vx, the switching transistor 1701, the driving transistor 1702, the capacitor 1703 and the light-emitting element 1704.
A gate electrode of the switching transistor 1701 is connected to the gate signal line G, and one of a source electrode and a drain electrode thereof is connected to the source signal line S while the other is connected to a gate electrode of the driving transistor 1702 and to one electrode of the capacitor 1703. One of a source electrode and a drain electrode of the driving transistor 1702 is connected to the power source line V while the other is connected to an anode or a cathode of the light-emitting element 1704. One of the two electrodes of the capacitor 1703 which is not connected to the driving transistor 1702 nor the switching transistor 1701 is connected to the power source line V.
Description is made below on the operation of a pixel having the aforementioned configuration where the light-emitting element 1704 emits light.
Upon input of a signal to the gate signal line G, the switching transistor 1701 is turned on. Through the source electrode and the drain electrode of the switching transistor 1701 which is on, a signal is inputted from the source signal line S to the gate electrode of the driving transistor 1702. The capacitor 1703 holds the potential of the source signal line S. By a signal inputted to the gate electrode of the driving transistor 1702, the driving transistor 1702 is turned on. At this time, a current value flowing between the source electrode and the drain electrode of the driving transistor 1702 is determined by a potential difference between the gate electrode of the driving transistor 1702 and the power source line V. When a current flowing between the source electrode and the drain electrode of the driving transistor 1702 flows into the light-emitting element 1704 through a pixel electrode of the light-emitting element 1704, the light-emitting element 1704 emits light.
At this time, the current value supplied to the light-emitting element 1704 is required to be constant at all times without being affected by the degradation of the light-emitting element 1704. The current value supplied to the light-emitting element 1704 is set constant independently of the potential difference between the source electrode and the drain electrode of the driving transistor 1702; therefore, the driving transistor 1702 is desirably designed to operate in the saturation region.
In this manner, in a conventional display, a forward driving voltage is applied to a light-emitting element.
However, it has been found that the degradation of the I-V characteristics of a light-emitting element can be improved by applying a reverse driving voltage to a light-emitting element at regular intervals (see Non-patent Document 1).
[Non-patent Document 1]
D. Zou et al., “Improvement of Current-Voltage Characteristics in Organic Light Emitting Diodes by Application of Reversed-Bias Voltage”, Jpn. J. Appl. Phys. Vol. 37 (1998), pp. L1406-L1408, Part 2, No. 11B, 15 Nov. 1998
There is an initial defect that a pixel electrode and a counter electrode are short-circuited, which produces a non-light-emitting region in the pixel. The short circuit may be caused due to the adhesion of foreign substances; pinholes in a thin electroluminescent layer which are produced by minute projections of an anode during the formation thereof; or pinholes which are produced due to the uneven deposition of a thin electroluminescent layer. In a pixel where such an initial defect occurs, light emission/non-light emission in accordance with signals is not performed and favorable image display cannot be performed because the whole elements cannot emit light with almost all currents flown to the short-circuit portion, or only specific pixels emit light or no light.
Not only such an initial defect, but another defect called a progressive defect may occur where the anode and the cathode are short-circuited with time. The short circuit between the anode and the cathode which is caused with time occurs due to the minute projections produced in the formation of the anode. That is, a stack having a pair of electrodes and an electroluminescent layer interposed therebetween has a potential short-circuit portion, which becomes dominant with time. It is said that in addition to the short circuit between the anode and the cathode, the progressive defect may be caused by a loose contact between the electroluminescent layer and the cathode which is caused by a slight gap between the electroluminescent layer and the cathode expanding with time.
The progress of the aforementioned initial defect can be suppressed by applying a reverse driving voltage to the light-emitting element to carbonize or oxidize the short-circuit portion to be insulated. In addition, the generation and progress of the aforementioned progressive defect can be suppressed by applying a reverse driving voltage to the light-emitting element to insulate the short-circuit portion by carbonization or oxidization, or by suppressing the expansion of the gap between the electroluminescent layer and the cathode.
However, in order to insulate the short-circuit portion, a sufficiently large current is required to be flown to insulate the short-circuit portion. Generally, a current which is sufficiently large to insulate the short-circuit portion has a far larger value than a forward current which is flown to the light-emitting element to emit light. In the pixel configurations in FIGS. 16 and 17, the current value supplied to the light-emitting element 1704 is controlled by the driving transistor 1702 in either case of the forward direction or the reverse direction. Provided that the current value flowing between the source electrode and the drain electrode, when the driving transistor 1702 is operated in the saturation region, is designed to be a forward current flowing to the light-emitting element 1704, the driving transistor 1702 cannot supply a sufficiently large current for insulating the short-circuit portion when a reverse driving voltage is applied to the light-emitting element.